Profile: keep the test's busy loop out of the vDSO (#63057)
The Profile tests collect their samples by spinning on `time_ns()`, so
nearly every sample is taken inside the kernel's vDSO `clock_gettime`.
On some kernels (Ubuntu's arm64 builds, for instance) that function
calls an out-of-line helper, and since the vDSO ships without unwind
tables and has no backing file for symbol lookup, libunwind's frame
record fallback cannot get past it: the backtrace ends after the two
vDSO frames. When every sample ends up like that, the profile contains
no Julia frames at all, and the printing tests that hide C frames fail
on an empty report. Spin in Julia code between clock reads instead, so
the samples land in JIT-compiled frames that unwind fine.
Assisted-by: Claude Code (Fable 5.1)
